The present invention relates to a process for the conversion of carbon monoxide and hydrogen (synthesis gas) to liquid hydrocarbon products in the presence of a Fischer-Tropsch catalyst.
In the Fischer-Tropsch reaction a gaseous mixture of carbon monoxide and hydrogen is reacted in the presence of a heterogeneous catalyst to give a hydrocarbon mixture having a relatively broad molecular weight distribution. This product is predominantly straight chain, saturated hydrocarbons which typically have a chain length of more than 5 carbon atoms. The reaction is highly exothermic and therefore heat removal is one of the primary constraints of all Fischer-Tropsch processes. This has directed commercial processes away from fixed bed operation to slurry systems. Such slurry systems employ a suspension of catalyst particles in a liquid medium thereby allowing both the gross temperature control and the local temperature control (in the vicinity of individual catalyst particles) to be significantly improved compared with fixed bed operation.
Fischer-Tropsch processes are known which employ slurry bubble columns in which the catalyst is primarily distributed and suspended in the slurry by the energy imparted from the synthesis gas rising from the gas distribution means at the bottom of the slurry bubble column as described in, for example, U.S. Pat. No. 5,252,613.
The Fischer-Tropsch process may also be operated by passing a stream of the liquid medium through a catalyst bed to support and disperse the catalyst, as described in U.S. Pat. No. 5,776,988. In this approach the catalyst is more uniformly dispersed throughout the liquid medium allowing improvements in the operability and productivity of the process to be obtained.
However, there remains the need for further improvements in the mode of operation of a Fischer-Tropsch process.
The present invention relates to a process for the conversion of gaseous reactants to liquid hydrocarbon products by contacting the gaseous reactants at an elevated temperature and pressure with a suspension comprising catalyst suspended in a liquid medium, in a system comprising a high shear mixing zone and a post mixing zone wherein the process comprises:
a) passing the suspension comprising catalyst suspended in the liquid medium through the high shear mixing zone where a gaseous reactant stream comprising the gaseous reactants is mixed with the suspension;
b) discharging a mixture comprising gaseous reactants and suspension from the high shear mixing zone into the post mixing zone;
c) converting at least a portion of the gaseous reactants to liquid hydrocarbon products in the post mixing zone to form a product suspension comprising catalyst suspended in the liquid medium and the liquid hydrocarbon products;
d) separating a gaseous stream comprising unconverted gaseous reactants from the product suspension;
e) recycling the separated gaseous stream to the high shear mixing zone; and
f) recycling at least a portion of the product suspension to the high shear mixing zone.
An advantage of the process of the present invention over conventional Fischer-Trospch processes is that enhanced mass transfer in the high shear mixing zone and the post mixing zone improves the contact between the gaseous reactants, liquid medium and solid catalyst and hence promotes the catalytic conversion of the gaseous reactants to liquid hydrocarbon products. For avoidance of doubt, the conversion of the gaseous reactants to liquid hydrocarbon products is initiated in the high shear mixing zone although the majority of the conversion generally occurs in the post mixing zone.
Preferably, the gaseous reactants comprise a mixture of carbon monoxide and hydrogen (synthesis gas). Preferably, the ratio of hydrogen to carbon monoxide in the synthesis gas is 2:1 by volume.
The synthesis gas may be prepared using any of the processes known in the art including partial oxidation of hydrocarbons, steam reforming, and autothermal reforming. A discussion of these synthesis gas production technologies is provided in xe2x80x9cHydrocarbon Processingxe2x80x9d V78, N.4, 87-90, 92-93 (April 1999) and xe2x80x9cPetrole et Techniquesxe2x80x9d, N.415, 86-93 (July-August 1998). It is also envisaged that the synthesis gas may be obtained by catalytic partial oxidation of hydrocarbons in a microstructured reactor as exemplified in xe2x80x9cIMRET 3: Proceedings of the Third International Conference on Microreaction Technologyxe2x80x9d, Editor W Ehrfeld, Springer Verlag, 1999, pages 187-196. Alternatively, the synthesis gas may be obtained by short contact time catalytic partial oxidation of hydrocarbonaceous feedstocks as described in EP 0303438. Preferably, the synthesis gas is obtained via a xe2x80x9cCompact Reformerxe2x80x9d process as described in xe2x80x9cHydrocarbon Engineeringxe2x80x9d, 2000, 5, (5), 67-69; xe2x80x9cHydrocarbon Processingxe2x80x9d, 79/9, 34 (September 2000); xe2x80x9cToday""s Refineryxe2x80x9d, 15/8, 9(August 2000); WO 99/02254; and WO 200023689.
Preferably, the liquid hydrocarbon products comprise a mixture of hydrocarbons having a chain length of greater than 5 carbon atoms. Suitably, the liquid hydrocarbon products comprise a mixture of hydrocarbons having chain lengths of from 5 to about 90 carbon atoms. Preferably, a major amount, for example, greater than 60% by weight, of the hydrocarbons have chain lengths of from 5 to 30 carbon atoms.
Suitably, the liquid medium comprises one or more of the liquid hydrocarbon products which has the advantage that there is no requirement to separate the liquid hydrocarbon products from the liquid medium.
The high shear mixing zone may be part of the system inside or partially outside the post mixing zone, for example, the high shear mixing zone may project through the walls of the post mixing zone such that the high shear mixing zone discharges its contents into the post mixing zone. The system may comprise a plurality of high shear mixing zones, preferably up to 250 high shear mixing zones, more preferably less than 100, most preferably less than 50, for example 10 to 50 high shear mixing zones. Preferably, the plurality of high shear mixing zones discharge into a single post mixing zone which has an advantage of significantly reducing the size of a commercial Fischer-Tropsch plant. Preferably, the plurality of high shear mixing zones may be spaced uniformly inside or partially outside the post mixing zone, for example, the high shear mixing zones may be spaced uniformly at or near the top of the post mixing zone. Preferably, the high shear mixing zones discharge the mixture of gaseous reactants and suspension in a downwards direction into the post mixing zone.
The high shear mixing zone(s) may comprise any device suitable for intensive mixing or dispersing of a gaseous stream in a suspension of solids in a liquid medium, for example, a rotor-stator device or an injector-mixing nozzle.
The injector-mixing nozzle(s) can advantageously be executed as venturi tubes (c.f. xe2x80x9cChemical Engineers"" Handbookxe2x80x9d by J. H. Perry, 3rd edition (1953), p. 1285, FIG. 61), preferably an injector mixer (c.f. xe2x80x9cChemical Engineers"" Handbookxe2x80x9d by J H Perry, 3rd edition (1953), p 1203, FIG. 2 and xe2x80x9cChemical Engineers"" Handbookxe2x80x9d by R H Perry and C H Chilton 5th edition (1973) p 615, FIGS. 6-31) or most preferably as a liquid-jet ejector (c.f. xe2x80x9cUnit Operationsxe2x80x9d by G G Brown et al, 4th edition (1953), p. 194, FIG. 210). Alternatively, the injector-mixing nozzle(s) may be executed as xe2x80x9cgas blastxe2x80x9d or xe2x80x9cgas assistxe2x80x9d nozzles where gas expansion is used to drive the nozzle (c.f. xe2x80x9cAtomisation and Spraysxe2x80x9d by Arthur H Lefebvre, Hemisphere Publishing Corporation, 1989). Where the injector-mixing nozzle(s) is executed as a xe2x80x9cgas blastxe2x80x9d or xe2x80x9cgas assistxe2x80x9d nozzle, the suspension of catalyst is fed to the nozzle at a sufficiently high pressure to allow the suspension to pass through the nozzle while the gaseous reactant stream is fed to the nozzle at a sufficiently high pressure to achieve high shear mixing within the nozzle.
Suitably, the gaseous reactant stream is fed to the high shear mixing zone at a pressure of at least 20 bar, preferably at least 30 bar. Typically, the pressure drop of the suspension over the high shear mixing zone is in the range of from 1 to 6 bar, preferably 2 to 5 bar, more preferably 3 to 4 bar. An advantage of the process of the present invention is that where the gaseous reactant stream comprises synthesis gas obtained via a xe2x80x9cCompact Reformerxe2x80x9d process, the synthesis gas is generally at a pressure of above 20 bar. Accordingly, there is no requirement to lower the pressure of the synthesis gas before feeding the synthesis gas to the process of the present invention thereby providing an energy efficient integrated Reforming/Fischer Tropsch process. In particular, the pressure of synthesis gas obtained via a xe2x80x9cCompact Reformerxe2x80x9d process is generally sufficiently high to achieve high shear mixing within a xe2x80x9cgas blastxe2x80x9d or xe2x80x9cgas assistxe2x80x9d nozzle.
Suitably, the shear forces exerted on the suspension in the high shear mixing zone(s) are sufficiently high that the gaseous reactant stream is broken down into gas bubbles having diameters in the range of from 30xcexc to 10 mm, preferably from 30xcexc to 3000xcexc, more preferably from 30xcexc to 300xcexc.
Preferably, the product suspension which is recycled to the high shear mixing zone (hereinafter referred to as xe2x80x9csuspension recycle streamxe2x80x9d) is cooled outside of the high shear mixing zone and the post mixing zone, in order to assist in the removal of exothermic heat of reaction from the system, for example, by passing the suspension recycle stream through a heat exchanger. Preferably, the suspension recycle stream is cooled to a temperature of not more than 12xc2x0 C. below the temperature of the suspension in the post mixing zone.
Preferably, additional cooling is provided within the post mixing zone by means of a heat exchanger, for example, heat transfer tubes, positioned within the suspension in the post mixing zone.
The gaseous stream comprising unconverted gaseous reactants may be separated from the product suspension either within the post mixing zone or in an external gas liquid separation zone. The separated gaseous stream may comprise vaporized low boiling liquid hydrocarbon products, vaporized water by-product and gaseous hydrocarbons having from 1 to 3 carbon atoms such as methane, ethane and propane, in addition to unconverted gaseous reactants.
The separated gaseous stream (hereinafter referred to as xe2x80x9cgaseous recycle streamxe2x80x9d) may be cooled before being recycled to the high shear mixing zone, for example, by passing the gaseous recycle stream through a heat exchanger, to assist in the removal of the exothermic heat of reaction from the system. Where the gaseous recycle stream is cooled to below its dew point, any vaporized low boiling liquid hydrocarbon products and any vaporized water by-product will condense out of the gaseous recycle stream and these condensed liquids are preferably removed from the system using a suitable separation means, for example, the heat exchanger may be fitted with a liquid trap. Water by-product may then be separated from the condensed low boiling liquid hydrocarbon products using a suitable separation means, such as a decanter. The low boiling hydrocarbon products may then be recycled to the high shear mixing zone and/or the post mixing zone. Fresh gaseous reactants may be fed to the gaseous recycle stream, either upstream or downstream of the heat exchanger. Where the fresh gaseous reactants have not been pre-cooled, it is preferred that the fresh gaseous reactants are fed to the gaseous recycle stream upstream of the heat exchanger. Preferably, the gaseous stream which is recycled to the high shear mixing zone comprises from 5 to 50% by volume of fresh gaseous reactants.
Preferably, a purge stream is taken from the gaseous recycle stream to prevent accumulation of gaseous by-products, for example, methane, in the system. If desired, any gaseous intermediate products (gaseous hydrocarbons having 2 or 3 carbon atoms) may be separated from the purge stream. Preferably, such gaseous intermediate products are recycled to the system where they may be converted to liquid hydrocarbon products.
Preferably, a stream comprising low boiling hydrocarbon(s) (for example pentanes, hexanes or hexenes) may be introduced into the high shear mixing zone and/or the post mixing zone. Without wishing to be bound by any theory, it is believed that vaporisation of the low boiling hydrocarbon(s) (hereinafter referred to as xe2x80x9clow boiling solventxe2x80x9d) in the high shear mixing zone and/or the post mixing zone aids and enhances the mixing of the gaseous reactants, liquid medium and the solid catalyst thereby increasing conversion of the gaseous reactants to liquid hydrocarbon products. Moreover, vaporisation of the low boiling solvent will also assist in removing some of the exothermic heat of reaction thereby allowing more control over the product selectivities and minimising the production of gaseous by-products, for example, methane. For avoidance of doubt, it is envisaged that the low boiling solvent may vaporise in both the post mixing zone and the high shear mixing zone. The gaseous recycle stream may therefore comprise vaporized low boiling solvent in addition to vaporized low boiling liquid hydrocarbon products, vaporized water by-product, unconverted gaseous reactants and gaseous hydrocarbons having from 1 to 3 carbon atoms. As discussed above, the gaseous recycle stream may be cooled before being recycled to the high shear mixing zone. Any vaporized low boiling solvent may condense, together with any vaporized low boiling liquid hydrocarbon products and any vaporized water by-product, upon cooling the gaseous recycle stream to below its dew point. Preferably, the condensed liquids are removed from the system, as described above, and water by-product may then be separated from the condensed liquids using a suitable separation means, also as described above. The remaining condensed liquids may then be recycled to the high shear mixing zone and/or the post mixing zone.
For practical reasons the post mixing zone may not be totally filled with suspension during the process of the present invention so that above a certain level of suspension a gas cap containing unconverted gaseous reactants is present in the top of post mixing zone. Suitably, the volume of the gas cap is not more than 40%, preferably not more than 30% of the volume of the post mixing zone. The high shear mixing zone may discharge into the post mixing zone either above or below the level of suspension in the post mixing zone. An advantage of the high shear mixing zone discharging below the level of suspension is that this improves the contact between the gaseous reactants and the suspension in the post mixing zone.
Where the post mixing zone has a gas cap, the gaseous recycle stream may be withdrawn from the gas cap. It is also envisaged that the post mixing zone may be fitted with an overhead condenser or cooler for removal of heat from the gases in the gas cap. Where the post mixing zone is fitted with an overhead condenser or cooler, the gaseous recycle stream may be withdrawn from the overhead condenser or cooler (i.e. is withdrawn indirectly from the post mixing zone). Any low boiling liquid hydrocarbon products and low boiling solvent which condense in the condenser or cooler may be collected and recycled to the high shear mixing zone or the post mixing zone (after having separated any water by-product).
The catalyst which may be employed in the process of the present invention is any catalyst known to be active in Fischer-Tropsch synthesis. For example, Group VIII metals whether supported or unsupported are known Fischer-Tropsch catalysts. Of these iron, cobalt and ruthenium are preferred, particularly iron and cobalt, most particularly cobalt.
A preferred catalyst is supported on an inorganic refractory oxide. Preferred supports include silica, alumina, silica-alumina, the Group IVB oxides, titania (primarily in the rutile form) and most preferably zinc oxide. The supports generally have a surface area of less than about 100 m2/g, preferably less than 5 m2/g, more preferably less than 25 m2/g, for example, about 5 m2/g.
The catalytic metal is present in catalytically active amounts usually about 1-100 wt %, the upper limit being attained in the case of iron based catalysts, preferably 2-40 wt %. Promoters may be added to the catalyst and are well known in the Fischer-Trospch catalyst art. Promoters can include ruthenium, platinum or palladium (when not the primary catalyst metal), rhenium, hafnium, cerium, lanthanum and zirconium, and are usually present in amounts less than the primary catalytic metal (except for ruthenium which may be present in coequal amounts), but the promoter:metal ratio should be at least 1:10. Preferred promoters are rhenium and hafnium.
A further advantage of the process of the present invention is that intensive mixing of the gaseous reactant stream and the suspension of catalyst in the high shear mixing zone allows smaller catalyst particle sizes to be employed compared with a conventional slurry process. Thus, the catalyst may have a particle size of less than 50 microns, preferably less than 40 microns, for example, in the range 5 to 30 microns. In contrast, a conventional slurry process will typically employ a catalyst having a particle size of greater than 40 microns. Advantages of smaller catalyst particle sizes include reducing the selectivity of the process of the present invention to methane (a gaseous by-product) and also reducing the formation of heavier hydrocarbon products. Without wishing to be bound by any theory, it is believed that catalyst particles having the preferred particle size of less than 40 microns may be formed in situ in the system by attrition of larger sized catalyst particles, for example, by attrition of a catalyst having a particle size of greater than 50 microns.
Preferably, the suspension of catalyst discharged into the post mixing zone comprises less than 40% wt of catalyst particles, more preferably 10 to 30% wt of catalyst particles, most preferably 10 to 20% wt of catalyst particles.
In a preferred embodiment the process is carried out using an injector-mixing nozzle. It has been found that intensive mixing of the gaseous reactant stream, the liquid medium and the solid catalyst can be achieved in the injector-mixing nozzle leading to high conversions of gaseous reactants to liquid hydrocarbon products in the post mixing zone. The suspension which is discharged by the injector-mixing nozzle into the post mixing zone is at least in part recycled to the injector-mixing nozzle, for example, via a slurry pump. The injector-mixing nozzle may draw in the gaseous reactant stream through at least one opening in its side wall (a venturi nozzle). Alternatively, as described above, the gaseous reactant stream may be supplied at high pressure to the injector-mixing nozzle through at least one opening in its side wall (a xe2x80x9cgas blastxe2x80x9d or xe2x80x9cgas assistxe2x80x9d nozzle). An advantage of using a xe2x80x9cgas blastxe2x80x9d or xe2x80x9cgas assistxe2x80x9d nozzle as the high shear mixing zone is that there is a reduced duty on the slurry pump.
More than one injector-mixing nozzle, preferably up to 150, more preferably less than 100, most preferably less than 50, for example 10 to 50 injector-mixing nozzles may discharge into a single post mixing zone.
Suitably, the post mixing zone comprises a vessel, for example, a tank reactor or a tubular loop conduit and the injector-mixing nozzle can be placed at any position on the walls of the vessel (for example, at the top, bottom or side walls of a tank reactor).
Where the vessel of the post mixing zone is a tank reactor, product suspension is withdrawn from the tank reactor and is at least in part recycled to the injector-mixing nozzle(s). Very good mixing can be achieved when the injector-mixing nozzle(s) is situated at the top of the tank reactor and the suspension is removed from the tank reactor at its bottom. Therefore the tank reactor is preferably provided at its top with at least one injector-mixing nozzle and the suspension recycle stream is preferably withdrawn from the bottom of the tank reactor. Preferably, the suspension recycle stream is at least in part recycled via a loop conduit (slurry recycle line) to the top of the injector-mixing nozzle(s) through which it is then injected into the top of the tank reactor, the gaseous reactant stream being introduced through one or more openings in the side wall of the injector-mixing nozzle(s). Preferably, a heat-exchanger is positioned on the loop conduit to remove the heat of reaction.
Where the vessel of the post mixing zone is a tubular loop conduit, a single injector-mixing nozzle may discharge into the tubular loop conduit. Suspension may be recycled to the injector-mixing nozzle, for example, via a pump or propeller positioned in the tubular loop conduit. A heat exchanger may be disposed along at least part of the length of the tubular loop conduit, preferably along substantially the entire length of the tubular loop conduit thereby providing temperature control. Alternatively, a series of injector-mixing nozzles may be arranged around the tubular loop conduit. In this arrangement each injector-mixing nozzle discharges into a section of the tubular loop conduit which section recycles the suspension to the next injector-mixing nozzle in the loop, for example, via a pump or propeller positioned in the section of the tubular loop conduit. A heat exchanger may be disposed along at least part of each section of tubular loop conduit, preferably along substantially the entire length of each section of tubular loop conduit thereby providing temperature control. It is envisaged that mixing of the gaseous reactants and the suspension of catalyst in the tubular loop conduit may be so efficient that there is no requirement for a gas cap. Where a gas cap is omitted, product suspension together with entrained and/or dissolved gases (unconverted gaseous reactants, gaseous hydrocarbons having from 1 to 3 carbon atoms, vaporized low boiling liquid hydrocarbon products, vaporized water by-product and optionally vaporized low boiling solvent) is withdrawn from the tubular loop conduit and a gaseous recycle stream comprising the entrained and/or dissolved gases is separated from the product suspension in an external gas liquid separation zone.
Where the vessel of the post mixing zone (e.g. tank reactor or tubular loop conduit) has a gas cap, advantageously the gaseous recycle stream is withdrawn through the vessel wall from the gas cap and is recycled to the injector-mixing nozzle(s). As mentioned above, an advantage of recycling the gaseous reactants from the gas cap to the injector-mixing nozzle(s) is that in this manner the temperature of the suspension in the vessel can be advantageously controlled by cooling the gaseous recycle stream in a heat exchanger outside the high shear mixing zone and the vessel of the post mixing zone. This temperature control can be further improved if fresh gaseous reactants are added to the gaseous recycle stream before it is cooled (upstream of the heat exchanger) or are pre-cooled. The temperature of the suspension in a tank reactor can also be controlled by means of a heat exchanger, for example, heat transfer tubes, positioned below the level of suspension in the tank reactor and by means of external cooling of the suspension recycle stream.
The process of the invention is preferably carried out at a temperature of 180-280xc2x0 C., more preferably 190-240xc2x0 C.
The process of the invention is preferably carried out at a pressure of 5-50 bar, more preferably 15-35 bar, generally 20-30 bar.
The process of the present invention can be operated in batch or continuous mode, the latter being preferred.
In a continuous process part of the product suspension is continuously removed from the system and is passed to a suitable separation means, where liquid medium and liquid hydrocarbon products are separated from the catalyst. Examples of suitable separation means include hydrocyclones, filters, gravity separators and magnetic separators. Alternatively, the liquid medium and liquid hydrocarbon products may be separated from the catalyst by distillation. The separated liquids are then passed to a product purification stage where water by-product and liquid medium are removed from the liquid hydrocarbon products. As discussed above, the purification stage may be simplified by using one or more of the liquid hydrocarbon products as the liquid medium in which case there is no requirement to separate the liquid medium from the liquid hydrocarbon products. The catalyst may be recycled as a concentrated slurry to the post mixing zone. Fresh catalyst may be added either to the recycled slurry or directly into the post mixing zone.
In order to prevent the accumulation of water by-product in the system it is preferred that at least a portion of the water by-product is removed from the suspension recycle stream. This may be achieved by taking a side stream from the suspension recycle stream downstream of the heat exchanger. The liquid components of the side stream are separated from the catalyst (as described above) and water by-product is removed from the separated liquids (also as described above) before recycling the remaining separated liquid components back to the high shear mixing zone. The separated catalyst may be recycled to the post mixing zone as a concentrated slurry (as described above).
It is envisaged that removal of water by-product from the system can be incorporated into the product separation stage, by recycling a portion of the separated liquids, from which water has been removed, back to the high shear mixing zone.
The liquid hydrocarbon products from the purification stage may be fed to a hydrocracking stage, for example, a catalytic hydrocracking stage which employs a catalyst comprising a metal selected from the group consisting of cobalt, molybdenum, nickel and tungsten supported on a support material such as alumina, silica-alumina or a zeolite. Preferably, the catalyst comprises cobalt/molybdenum or nickel/molybdenum supported on alumina or silica-alumina. Suitable hydrocracking catalysts include catalysts supplied by Akzo Nobel, Criterion, Chevron, or UOP. A preferred catalyst is KF 1022(trademark), a cobalt/molybdenum on alumina catalyst, supplied by Akzo Nobel.